In the signal path of a radio transmitter, two blocks which handle the coding and the modulation of the transmit signals can be distinguished by their function. These are called an outer transmitter and an inner transmitter. The outer transmitter, by coding the transmit data, generates bits dk which are modulated onto a carrier frequency by the inner transmitter and are then transmitted by means of an antenna.
In corresponding manner to the radio transmitter, a radio receiver has an inner receiver and an outer receiver. In the inner receiver, the received radio signals are demodulated and converted into softbits sk. The softbits sk, which are still coded, are then decoded by the outer receiver.
In consequence, the bits dk generated by the outer transmitter in the radio transmitter are reconstructed in the form of softbits sk with the aid of the inner receiver. The mapping of the bits dk onto the softbits sk, and thus the quality of the softbits, is significantly determined by the quality of the inner receiver, apart from the transmission characteristics and the characteristics of the radio-frequency front end of the radio receiver.
Depending on the radio transmission system and, in particular, on the outer receiver which processes the softbits further, various measures can be used for assessing the quality of the softbits and of the inner receiver generating the softbits. For example, the softbits can be assessed by means of the distortion to which they are subjected by the quantizing. Since, as a rule, the softbits are present as fixed-point numbers in conventional radio receivers, the number of bits used for representing numbers contributes decisively to the quality of the softbits. The quantization noise can be reduced to below any threshold of greater than zero by means of a sufficiently large number of bits. However, critical cost factors of a digital integrated circuit such as the chip area needed or the energy consumption are also increased with the bit width.